The present invention relates generally to a method and an apparatus for controlling an electromagnetic clutch for use on a vehicle. More particularly, the invention is concerned with improved technologies for effecting a smooth engaging action of such an electromagnetic clutch upon starting of the vehicle, while assuring a satisfactorily high fuel economy of the vehicle.
In the art of a power transmission system of a vehicle, an electromagnetic clutch is used as means for easy control of a torque to be transmitted. An example of such an electromagnetic clutch comprises: a driving rotary member operatively connected to an engine of a vehicle; a driven rotary member operatively connected to drive wheels of the vehicle and cooperating with the driving rotary member to define an annular space therebetween; a mass of magnetic powder accommodated in the annular space between the driving and driven rotary members; and a solenoid which is energized to generate a magnetic force for causing particles of the magnetic powder to be tightly bonded together in the annular space so that the mass of magnetic powder connects the driving and driven rotary members to transmit a torque from the driving rotary member to the driven rotary member. In this electromagnetic clutch, a value of the torque which is delivered from the driving rotary member (from the engine) to the driven rotary member (drive wheels) is varied as a function of an electric current to be applied to the solenoid. This characteristic of the electromagnetic clutch is utilized to obtain an optimum or smooth control of an engaging action of the clutch, for example at the time of starting the vehicle, by increasing a magnitude of solenoid energizing electric current at a predetermined rate relative to the engine speed. After the engine speed has been raised to a predetermined level, the solenoid energizing current is made constant.
However, it has been a common practice that such a solenoid energizing current is increased at a fixed rate in proportion to the actual speed of the engine. Further, the rate of increase in the energizing current relative to the engine speed is generally set at a relatively low level, so that the clutch engagement is effected slowly to provide sufficiently high drivability of the vehicle even when the vehicle is started or accelerated abruptly or started on an upward slope. This low setting of the increase rate of the solenoid energizing current (i.e., of the torque transmitted by the clutch) relative to the engine speed, does not necessarily assure satisfactory fuel economy (low specific fuel consumption) while the engine is operating for starting the vehicle.